1. Technical Field
This invention relates generally to electronic circuits and in particular to a circuit which provides a differential pair of current signals.
2. State of the Art
In some electronic devices it is desirable to implement a current signal, where the level of current is adjustable throughout a range of current levels. In some designs a differential pair of current output signals is desirable. Using a differential pair of signals provides a circuit with greater dynamic range and noise immunity. It is also desirable in some situations to have a current output signal which is linearly proportional to an input voltage signal throughout a voltage range. This allows the output current level to be accurately adjusted over time by adjusting the input voltage range.
A resistor circuit can be used to create a current signal that is linearly proportional to a voltage, but this does not fulfill the need for a differential pair of current output signals that are linearly proportional to an input voltage. A pair of source-coupled transistors can be used to create a differential pair of current output signals, but the differential pair of current output signals have a relatively small voltage operating range over which they are linearly related to the input voltage. Thus it is desirable to have an electronic circuit which generates a differential pair of current output signals that are linearly related to an input voltage signal within a wide range of input voltages.
Additionally, it is desirable to generate the differential pair of current output signals with a circuit that provides a repeatable and stable level of output current regardless of process or temperature variations. In some circuits the voltage operating range is offset from its desired values due to process and temperature variations. In other circuits the variation in the resistance value of a resistor due to process or temperature variations can cause variations in the amplitude of output signals. Voltage offset and/or variations in signal output can cause the system that the electronic circuit is a part of to become unstable or to change states or levels undesirably. Thus it is desirable to have an electronic circuit that outputs a differential pair of current output signals that are linearly proportional to a voltage input signal, where the signal output levels are stable over process and temperature variations.